A: In the early days of VDI those were your only options. But these days you can find VDI solutions from many vendors, some of which you might be familiar with and others you likely aren't. One of the biggest differences between the VDI solutions from the first-party platform vendors and those from third parties has to do with their level of integration.
Take Citrix, for example. Their XenDesktop VDI solution is designed to integrate directly with their other application delivery solutions, such as presentation or application virtualization, using Citrix XenApp. Using their multifaceted approach, the way users access applications or desktops can be tailored to their needs—you could deliver some applications via presentation, others through streaming, and others via desktop virtualization.
In contrast with this approach, many third-party VDI vendors focus on a single delivery mechanism. One example, Wanova, maintains a virtual desktop at the client's location that remains synchronized with a centralized image. NComputing's solution lets multiple users to simultaneously share a single computer. Other lesser-known examples include products from MokaFive, Parallels, and Quest.
Obviously, everyone's requirements for what they need from a VDI solution vary. However, one potential attractor to third-party solutions can be complexity. While the single delivery mechanism approach used by third party solutions might lack the integrations of the platform vendors, getting their technologies implemented might be an easier, simpler, or less expensive project overall.